Fluids can contain suspended particles of many different sizes. Some suspended material will be large enough and heavy enough to settle rapidly to the bottom if a liquid sample is left to stand. Very small particles settle very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid or murky.

In drinking water, the higher the turbidity level, the higher the risk that people may develop diseases.[3] Contaminants like viruses or bacteria can become attached to the suspended solids. The suspended solids interfere with water disinfection with chlorine because the particles act as shields for the virus and bacteria. Similarly, suspended solids can protect bacteria from ultraviolet (UV) sterilization of water.[source?]